ftruncate may not be supported

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Description

Sue LoVerso, one of the groups interested in the pluggable file system work has a file truncate that can extend a file, but not shorten the file.

The logging subsystem calls ftruncate in a few places, can you please describe when it's OK for ftruncate to fail to shorten the file and when it's OK for ftruncate to fail to extend the file?

Obviously, it would be great if it's never a matter of correctness for ftruncate to fail in either case.

FTR, the block manager also calls ftruncate, but it's never an issue of correctness, and it should be relatively easy to change the block manager to ignore systems that don't support any kind of ftruncate.